1. Field of the Invention
The present invention relates to a switching power supply device and a switching method used by the same.
2. Description of the Related Art
A conventional switching power supply (switched-mode power supply) device is disclosed in Unexamined Japanese Patent Application KOKAI Publication No. 2004-135415.
FIG. 13 shows a circuit diagram illustrating a conventional switching power supply device given in the Publication.
The switching power supply device comprises a main switching element Q1, a synchronous rectification switching element Q2, a series circuit 26, and a synchronous rectification switching control circuit 27.
The main switching element Q1 switches (turns on and off) a current which flows through the primary winding LP, of a transformer T1. The synchronous rectification switching element Q2 is connected between the secondary winding, LS, of the transformer T and a load. The series circuit 26 includes a synchronous-rectification-inductance element L1 and a diode D1, and is connected to the secondary winding LS of the transformer T1 in parallel. The synchronous rectification switching control circuit 27 includes a diode D2, a capacitor C51, and a transistor Q5.
The synchronous rectification switching element Q2 turns off in an on period of the main switching element Q1, and stores or accumulates electrical power in the transformer T1 and the synchronous-rectification-inductance element L1. In tern, the synchronous rectification switching element Q2 turns on in an off period of the main switching element Q1, and permits the stored electrical power to be released. Before the release of the electrical power stored in the transformer T1 is completed, the action of the diode D1 causes the synchronous-rectification-inductance element L1 to complete the release of the stored electrical power. In accordance with a voltage at a node A between the synchronous-rectification-inductance element L1 and the diode D1, the diode D2 in a synchronous rectification switching control circuit 27 detects that the stored electrical power of the synchronous-rectification-inductance element L1 has been released, and turns off the synchronous rectification switching element Q2.
In the switching power supply device, even if the release of the stored electric power of the transformer T1 is completed, the voltage at the node A may not drop instantaneously. To be more precise, the voltage at the node A may not drop instantaneously after the release of the stored electric power from the transformer T1 is completed because of the effect of the inductance of the synchronous-rectification-inductance element L1 and the capacity of the synchronous-rectification control circuit 27, or the parasitic capacity of the synchronous rectification inductance element L1. The delay of the reduction in voltage at the node A may keep the synchronous rectification switching element Q2 turned on after the release of the stored electrical power of the transformer T1 is completed. This may deteriorate the efficiency, and damage the element.